Despite efforts to replace leather with synthetic leather-like substitutes made from other fibers, sheet materials, and petrochemicals, leather still is the product of choice by consumers. Leather has unique properties that make it ideally suited for use in a variety of products. One of the most important properties of leather is its ability to conform with respect to shape. Plastic flow imparts the necessary give in articles of manufacture, while elastic flow ensures the proper recovery after the flexing action. Leather substitutes fail to achieve the combination of these properties, nor are they able to remove moisture from the surface in which they are in contact. The mechanism of water vapor transmission has been shown to be a function of the material rather than a linear diffusion process.
The leather forming process includes a number of steps before the leather is ready for use in the manufacture of products. Fresh cattlehides are first rehydrated to restore some lost moisture, after which they undergo a process commonly referred to as unhairing or cleaning and degreasing, whose object is the removal of hair, epidermis and certain soluble proteins. This is accomplished by contacting the hides in baths containing caustics, electrolytes, and other components typically employed in the industry. The chemicals used in this process serve three functions: to destroy the hair or attack the hair root so that it comes free from the hide; to loosen the epidermis, which is a hard outer layer covering the grain; and to remove certain soluble skin proteins that lie within the hide substance. The hides then undergo a number of mechanical processes which further remove undesirable parts of the hide. Residual unhairing chemicals, such as hydrated lime for example, and other non-leather making substances are further removed by a process known as bating. Bating chemicals typically come in contact with the hides in various types of vessels to accomplish this procedure. The next major process involved in the treating of leather hides is known as tanning, which stabilizes the collagen fibers present in the hides. There are two types of tanning processes, chrome and vegetable tanning. Prior to engaging in either process, the unhaired hides are acidified with sulfuric acid to lower their pH to less than 3. According to the chrome tanning process, the acidified hides are dipped in an aqueous solution of chromium sulfate until the solution has fully penetrated the hide and its pH has been raised. Once this step is completed, the collagen has been fully reacted and the hide preserved.
Vegetable tanning involves the same preparatory steps as that of chrome tanning except that here, the hides are soaked for a number of days in pits containing solutions of water-soluble extracts of various parts of plant materials, including leaves, fruits, pods, and roots. Vegetable tanning, unlike that of chromium, produces a fullness and resiliency characteristic of only this type of tannage. It provides improved molding characteristics and a hydrophilic character for enhanced removal of perspiration if worn against human skin. The tanned hides are then cleaned of their tannins, placed under pressure to pack the fibers of the leather, and aged.
After the hides are subjected to further preparatory mechanical processes, the remaining wet steps include retanning, coloring, and fatliquoring. Retanning is a wet process which imparts special end-use properties with other tanning chemicals. Chemicals such as vegetable extracts are used for adding solidity and body to the leather, syntans for levelling the color imparted to the leather, and mineral retanning agents for imparting softness to the leather. Coloring with aniline-derived, water-soluble dyes for aesthetic purposes is the next step. Fatliquoring is a process by which the fibers of the leather hide are lubricated in order to impart flexibility and softness to the leather. The leather is then dried, at which point it may be used as is for some applications. In general, however, drying renders the leather too hard and unworkable for most article manufacturing processes so that it must be conditioned. Conditioning or rehydrating involves the introduction of controlled amounts of moisture back into the leather. The final stage of leather treating involves the finishing process. It is the finishing process with its application of natural or synthetic polymers and colorants both within and on the surface of the leather that produces the uniformity, appearance characteristics, and resistance to scuffing and abrasion that are required for a commercial product.
The satisfactory application of the various wetting processes requires that a surfactant and/or a solvent be used as a component of the particular type of composition being employed. The surfactant and/or solvent acts to ensure that the particular composition to which it is added is evenly and effectively distributed throughout the leather. In other words, it aids in the penetration and absorption of the composition into the leather hides. Due to the volatile organic nature of both the solvents and surfactants used in the wet leather treating steps, the EPA has adopted strict guidelines regarding their use and disposal. Compositions which contain such solvents and surfactants are sometimes referred to as volatile organic compound (VOC) compositions. As a result, any solution containing such compositions is potentially dangerous to both its users and the environment. Leather treating solutions and processes which provide for the full or even partial replacement of these harmful components in compositions by more ecologically compatible or friendly components which perform the same function without detracting from the effectiveness of the particular process being employed are highly desirable.
Therefore, there is a need to provide a composition and process for treating leather hides containing a substantially VOC-free surfactant and/or solvent component, as well as a process for effectively treating leather hides in an environmentally friendly manner.
The present invention provides a composition and process for the rehydrating, cleaning and degreasing, and finishing processes of leather.